Ultrasound is a common means of nondestructively inspecting materials for flaws and structural integrity. For steels, the preferred frequency used for inspection and sizing of flaws is in the range of 1-10 MHz with 2.25-5 MHz preferred. Pulsed ultrasonic transducers and their electronics, common to the art of nondestructive examination (NDE), are typically relatively broad-band, with bandwidth roughly 40% of the nominal operating frequency. Such transducers and electronics are, therefore, ill-suited to interferometry, which requires narrow-band, or "monochromatic", continuous wave "bursts". In addition, the NDE acoustical paths are not critically related to inspection accuracy, except that long paths in metal reduce signal strength due to attenuation. In contrast, interferometry requires well-defined paths of accurate lengths. Therefore, standard methods commonly applied to NDE are not directly applicable to interferometry.
The basic principle behind any interferometer, whether of the acoustical, optical or microwave variety, is linear superposition of waves, i.e., two or more partial waves add algebraically to give the total wave amplitude. This is particularly simple when all of the partial waves have substantially the same frequency. Thus, a monochromatic source is especially desirable to optimize performance of an interferometer.